Power electronic circuits are used to control and condition electric power. For instance, power electronic circuits may be used to convert a direct current into an alternating current, to change voltage or current magnitude, or to change the frequency of an alternating current.
An inverter is a power electronic circuit which receives a dc source signal and converts it into an ac output signal. Harmonic neutralization and pulse-width modulation techniques are used to generate the ac signal. Harmonic neutralization involves a combination of several phase-shifted square-wave inverters, each switching at the fundamental frequency. Pulse-width modulation involves switching a single inverter at a frequency many times higher than the fundamental.
Filters can be classified according to whether their main purpose is to improve the power waveform or to remove EMI. Filters for waveform improvement usually deal with frequencies in the audio range. EMI filters are usually concerned with frequencies of 455 kHz or higher.
Passive filters are typically used to eliminate undesirable harmonics from the inverter output. Unfortunately, passive filters do not provide continuous harmonic filtering on pulsating or randomly varying loads. This occurs because passive filters only adapt to new harmonic levels after a considerable settling delay.
Passive filters tend to be large, heavy, costly, and, in general, highly load-dependent. Consequently, passive filters frequently represent a substantial part of the total cost, weight, and size of power electronics equipment.
Active filters represent an emerging technology without many of the shortcomings associated with passive filters. The technology relies upon the theory of active-feedback filters. A feedback loop and an inverter with a single energy-storage element (an inductor or capacitor) is used to minimize the difference between the actual waveform and the desired waveform.
The urgency of developing successful active power filters has recently grown in view of the increasing waveform distortion of both voltages and currents in ac power distribution systems. These distortions are largely attributable to a growing number of nonlinear loads in the electric utility power network. Typical nonlinear loads are computer controlled data processing equipment, numerical controlled machines, variable speed motor drives, robotics, medical and communication equipment.
Nonlinear loads draw square wave or pulse-like currents instead of purely sinusoidal currents drawn by conventional linear loads. As a result, nonlinear current flows through the predominantly inductive source impedance of the electric supply network. Consequently, a non-linear load causes load harmonics and reactive power to flow back into the power source. This results in unacceptable voltage harmonics and load interaction in the electric power distribution in spite of the existence of voltage regulators.
The degree of current or voltage distortion can be expressed in terms of the relative magnitudes of harmonics in the waveforms. Total Harmonic Distortion (THD) is one of the accepted standards for measuring voltage or current quality in the electric power industry.
Apart from voltage and current distortion, another related problem may arise when nonlinear loads are connected to the electric power network. In particular, when the load current contains large amounts of third or other triplen harmonics, the harmonic current tends to flow in the neutral conductor of the power system. Under these conditions, the neutral current can exceed the rated current of the neutral conductor. Since the neutral is normally designed to carry only a fraction of the line current, overheating or even electric fires can result.
As previously indicated, active filters may be used to alleviate these problems. Active filters, or active power line conditioners (APLCs) comprise one or two pulse width modulated inverters in a series, parallel, or series-parallel configuration. The inverters share a common dc link, which can be a dc inductor (current link) or a dc capacitor (voltage link). It is advantageous to keep the energy stored in the dc link (capacitor voltage or inductor current) at an essentially constant value. The voltage on the dc link capacitor can be regulated by injecting a small amount of real current into the dc link. The injected current covers the switching and conduction losses inside the APLC. The link voltage control can be performed by the parallel inverter.
The basic active load current compensation with current or voltage source filters is known. FIG. 1 depicts a parallel connected current source active filter 20, and FIG. 2 depicts a parallel connected voltage source active filter 22. The load current I.sub.L consists of three components: The real current, I.sub.r, the reactive current, I.sub.q, and the ripple current, I.sub.R. The parallel connected active filter supplies the I.sub.R and I.sub.q components, and, also, a small residual "high frequency" component I.sub.hf, that flows into the parallel connected "high frequency" capacitor C.sub.hf. The parallel connected active filter is essentially a single phase inverter which is operated from an isolated current or voltage source.
The realization of the active filter requires solid state switches with intrinsic turn-off capability (transistors, IGBTs, MOSFETs, GTOs, etc.). Switch pairs P1 and P2 are alternately turned ON or OFF. The average voltage required in the link capacitor, V.sub.dc, of FIG. 2, is supplied by the ac source. Real power can be absorbed by introducing an appropriate amount of offset in the symmetry of the on-times in switches P1 and P2. The polarity of the offset is coordinated with the polarity of the input voltage. When switches P1 of FIG. 2 are on, a current is generated between the tie inductor, Lp, the output capacitance dominated by C.sub.hf, and the difference between the dc link and ac output voltages. Conversely, when the P2 switch pair is on, the current is driven by the sum of the dc link and ac output voltages.
The real power, necessary to maintain the selected dc link voltage magnitude, Vdc, is proportional to the average duty cycle of high-frequency pole switchings in any given half line voltage cycle. The isolated dc link voltage is regulated by a closed loop controller that affects the average pole switching symmetry. Reactive inverter currents can be produced that flow in or out of the inverter by temporary changes in the duty cycle of inverter pole switchings. The instantaneous magnitudes of inverter currents are regulated so that they provide the load compensation current requirements. For example, if a positive ripple current is detected, the on-time of P2 is increased with respect to P1. The increase results in the required net compensating ripple current flowing in the ac line. This also implies that the amplitude of Vdc must be kept higher than the highest value of the ac voltage across the load, otherwise, the instantaneous compensation capability of the active filter is impaired.
The rapid pulse width modulation switching in the active filter produces a high frequency, triangular shaped current, I.sub.hf, an undesired side effect. The effect of the I.sub.hf signal is a small, superimposed triangular voltage ripple on the ac voltage. The amplitude of the voltage ripple is inversely proportional to the pole switching (carrier) frequency and the value of C.sub.hf. The voltage ripple is filtered with a parallel capacitor C.sub.hf.
When the active power filter (20 or 22) is connected across the load, a high degree of filtering of the terminal voltage is observed. Note that the active power filter is not capable of supplying or absorbing any real power other than that which is needed to compensate for losses inside the filter itself. It will, however, readily compensate reactive currents, non-synchronous and non-theoretical harmonics and sources with variable or unregulated frequency. The shunt connected power circuit is inherently protected under load short circuits since the load fault current bypasses the active power filter.
The isolated dc link circuits of FIGS. 1 and 2 can be combined to produce an ac line conditioner and voltage regulator. FIG. 3 depicts a shared link current source active power filter 24 with a serial inverter 26 and a parallel inverter 28. FIG. 4 depicts a shared link voltage source active power filter 30, with a serial inverter 32, and a parallel inverter 34. The respective series and parallel inverters are similar to the filters described in relation to FIG. 1 and 2. The shared link approach of FIGS. 3 and 4 represents a combination of series and shunt connected filters which are operated from a common shared direct voltage (or current) source.
The shared link circuit topology removes the former limitation of the active power filter, namely, that it is not capable of supplying or absorbing any real power, apart from compensating for the losses in the active power filter itself. In the shared dc link series and parallel circuit topology, it becomes possible for both the series and the parallel filter element to absorb or generate real power at the fundamental frequency, or other frequencies, provided the total power absorbed equals the total power generated.
The series active elements (26 and 32) may be modulated to provide a fundamental voltage of controllable magnitude and phase so that the phase and magnitude of the ac output voltage stays sinusoidal at any required level and phase angle with respect to the ac input. The power required by the series element (26 or 32) is absorbed from or injected into the dc link (36 or 38). Link energy is then maintained by appropriately controlling the phase and magnitude of the fundamental modulating signal applied to the parallel connected element (28 or 34). The result is that the power needed by the series element (26 or 32) will be obtained from the parallel element (28 or 34). Similarly, power generated by the series element (26 or 32) will be returned into the ac output by the parallel element (28 or 34).
When the output and input voltages are not equal, the series inverter (26 or 32) delivers real power to or from the dc link (26 or 38). The amount of power exchange delivered with respect to the output power depends on the fundamental Vo/Vin ratio. When the Vo/Vin ratio is smaller than unity, the real part of the input current becomes larger than the output (load) real current. The difference between the output and input currents flows through both inverters via the dc link (36 or 38). Appropriate fast-acting controls insure that the power flow between the series and parallel inverters is kept equal on the average, so that the power flow does not significantly alter the stored energy in the shared dc link.
In addition to the regulation of the buck/boost power transfer, the parallel active element (28 or 34) is modulated at ripple frequency so that it provides a bypass for load generated ripple currents and, if required, for the reactive fundamental current of the load. After full compensation of ripple and reactive components, only real fundamental current is drawn from the ac input.
The output signal of prior art active power line conditioners frequently includes amplitude and phase errors of the fundamental and of harmonics of the fundamental. Thus, it would be desirable to provide a controller which nulls amplitude and phase errors at selected frequencies. Conventional control techniques which rely upon proportional, integral, and/or derivative gains are inadequate to achieve these levels of waveform quality.